1. Field of the Invention
This invention relates to circuit protection devices comprising conductive polymers, particularly to improved circuit protection devices for use in protecting batteries.
2. Introduction to the Invention
Circuit protection devices for use in protecting batteries from overcurrent and overtemperature conditions are well-known. See, for example, U.S. Pat. Nos. 4,255,698 (Simon), U.S. Pat. No. 4,973,936 (Dimpault-Darcy et al), U.S. Pat. No. 5,801,612 (Chandler et al), U.S. Pat. No. 5,580,493 (Chu et al), and Japanese Utility Model Application No. 4-75287 (filed Oct. 29, 1992), the disclosures of which are incorporated herein by reference. In these applications, a device which exhibits a positive temperature coefficient of resistance (PTC behavior) is connected in series with a battery terminal. During normal operation the PTC device is in a low resistance, low temperature condition. When a very high current occurs, for example, due to a short circuit, or a very high temperature occurs, for example, during excessive charging, the device xe2x80x9cswitchesxe2x80x9d into a high resistance, high temperature condition, thus decreasing the current through the battery to a low level and protecting the battery and any components in electrical contact with the battery. The temperature at which this transition from low resistance to high resistance occurs is the switching temperature, Ts. Ts is defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of the PTC element as a function of the temperature which lie on either side of the portion of the curve showing a sharp change in slope.
Battery packs, in which a plurality of batteries, i.e. cells, are present, are commonly used with electrical equipment such as cameras, video recorders, tools, portable computers, personal data assistants (PDAs) and cellular phones. It is desirable to make the battery packs as small and lightweight as possible, but still provide adequate protection in the event of a short circuit, a runaway charge fault, charging at the wrong voltage, and/or reverse charging. One technique to maximize the use of space in the battery pack is to place the PTC device directly onto the button terminal of the battery, inside the battery pack. If the device is in the form of a disk with a central hole, the hole can be sized to allow it to be placed over the button terminal. Electrical connection is then made from an electrode on one surface of the disk to the button terminal and from an electrode on the opposite surface of the disk to a second battery. Alternatively, the PTC device can be in the form of a chip with attached straps. One strap is electrically connected to the button terminal of one battery and the other strap is attached to the second battery. Such an arrangement is useful in applying the device outside the battery pack.
While battery packs are used for many portable electronic devices, the battery packs used for cellular phones have special requirements. Due to the digital nature of such phones, battery packs are discharged in short bursts of high current. Because the circuit protection device is in series with the cells of the pack, there may be an unacceptable high voltage drop across the protection device if the device resistance in the unswitched state is too high. This attenuates the pulse and results in audio static as well as reduced battery capacity. In addition to requiring a very low resistance, i.e. less than 0.030 ohms, and preferably less than 0.025 ohms, it is desirable that the device have a low switching temperature, i.e. less than 100xc2x0 C., and preferably less than 90xc2x0 C., so that batteries with relatively long discharge times, e.g., nickel-metal hydride batteries and lithium-ion batteries, which are sensitive to overtemperature conditions switch at a temperature low enough to prevent damage to the battery itself or the case surrounding it. In addition to the very low resistance and very low switching temperature requirements, it is necessary that the device have sufficient PTC anomaly so that it can withstand the voltage required for the application, e.g. 8 to 24 volts. Thus an appropriate circuit protection device will provide recharging protection by protection in the event of overtemperature conditions, and will provide discharging protection by protecting in the event of overcurrent conditions. In addition, the device should be as small as possible in order to conserve space within the battery pack, an important consideration as equipment becomes smaller.
We have now found that by using a particular conductive polymer composition, an improved circuit protection device for battery pack protection against both overtemperature and overcurrent conditions can be made. The resulting devices have a low resistance and a low switching temperature, yet have a PTC anomaly of 103.5 or greater. Such devices allow improved protection of batteries used in portable electronic equipment because their low resistance results in minimal voltage drop and therefore will cause minimal distortion of signals and loss of battery capacity, and their very low switching temperature provides protection against overheating. The PTC anomaly of 103.5 or greater will provide sufficient voltage withstand for battery protection applications. These improvements in protection are achieved without an undesired decrease in current-carrying capacity under operating conditions.
Thus, in a first aspect, this invention provides a circuit protection device which comprises a circuit protection device which comprises
(A) a resistive element which is composed of a PTC conductive polymer composition which comprises
(1) a polymeric component comprising an ethylene/vinyl acetate copolymer having
(a) units derived from a first monomer which is ethylene,
(b) units derived from a second monomer which (i) is vinyl acetate, and (ii) comprises less than 30% by weight of the ethylene/vinyl acetate copolymer, and
(c) a melting point Tm of at most 105xc2x0 C., and
(2) dispersed in the polymeric component, a particulate conductive filler; and
(B) two electrodes which
(1) are attached to the resistive element, and
(2) can be connected to a source of electrical power,
said device having the following characteristics:
(i) a resistive element thickness of 0.025 to 0.25 mm;
(ii) a crosslinking level equivalent to 1 to 20 Mrads;
(iii) a surface area of at most 120 mm2;
(iv) a resistance at 20xc2x0 C., R20, of at most 0.050 ohm; and
(v) a PTC anomaly from 20xc2x0 C. to (Tm+5xc2x0 C.) of at least 103.5.
Such a device is particularly useful for protecting batteries. Therefore, in a second aspect, the invention provides an assembly which comprises
(I) a battery; and
(II) a circuit protection device of the first aspect of the invention which is in electrical contact with the battery.